Simply put, it's the ability to run (really run) user generated content. Gamasutra's Brandon Sheffield and Christian Nutt sat down with the Epic Games' mouthpiece to chat about all things Unreal—why Epic won't dedicate time to porting Unreal Engine 3 to Wii, why he's "very sensitive" about accusations that all UE3 games have a "look" and why the team decided to go to the PlayStation 3 for their latest shooter. Rein clears up some differences between user created content in Sony's LittleBigPlanet and Home which is decidedly non-"Wild West", almost anything goes when it comes to Unreal Tournament III mods.
Of course, Rein says they'll have a mechanism in place to blacklist certain mods that users report as objectionable, but the user created content on the PS3 version is the real deal. It's not just "rearranging chairs on the deck" a la LittleBigPlanet, it's scripting, graphics, models, level design.
Rein also chimes in, albeit briefly, about Square-Enix's plans with the Unreal Engine. Every sensitive about slights against his company's platform, he hints "there's a game that doesn't look anything like our games" coming from the Final Fantasy/Dragon Quest factory. Details are scarce, but Rein does let us know it's "neat." Consider it pre-ordered!
The Unreal Man: Mark Rein Speaks [Gamasutra]